The tube may especially be an amplifier, such as a TWT (travelling wave tube) or a klystron, for example. It may also be an oscillator (magnetron, etc.). Typically, it is desired to send the energy amplified inside the tube to a waveguide containing air. The microwave window allows the electromagnetic energy to pass freely to the waveguide, at least within a given frequency band, while still sealing against the vacuum inside the tube.
Conventionally, a window comprises a flat disk of insulating dielectric through which the electromagnetic energy passes. This disk is usually made of alumina or another ceramic having not only very good dielectric properties but also a high thermal conductivity and good resistance to high temperatures and to large temperature gradients. This is because, for high-power tubes operating with high electric fields, passage of the energy induces losses in the dielectric, hence substantial heating. The tubes in question here may provide power levels of several tens of kilowatts. Typically, the dielectric disk may have dimensions of some ten centimeters in diameter and a thickness of 1 mm to several millimeters.
To achieve a seal, the dielectric disk is brazed all around its periphery to the inner surface of a cylindrical skirt made of metal (generally copper) that surrounds it.
In a previous embodiment, shown in FIG. 1, the cylindrical skirt 10 is itself surrounded by a holder 20, for example made of stainless steel, serving as a support for fastening the dielectric disk 30 and its skirt 10 between the power tube and a waveguide. The dielectric disk 30 is brazed inside the skirt. The holder 20 may be used both as a heat sink and as a transition between the tube and the waveguide. It has an inner part 22 constituting the start of the waveguide, with a peripheral flange 24 used for fastening the waveguide to the holder 20. The upper part 26 of the holder 20 is cylindrical and surrounds the cylindrical skirt 10. This upper part is intended to be welded or brazed around an output port of the power tube (not shown). The bottom and the top of the skirt are brazed to the inside of the holder 20.
The brazed joints between the dielectric disk and the skirt and the brazed joints between the skirt and the holder contribute to maintaining a vacuum seal.
The thermal stresses in operation may be very high due to the high power dissipation occurring in the dielectric disk. The power dissipation is generally a maximum along the center axis and lower around the edges away from the center axis. The thermal conduction properties of the ceramic (especially alumina) allow the heat to be removed radially towards the edges; the copper skirt and the stainless steel holder serve as a heat sink. Despite this radial removal of the heat, the thermal stresses are very high because of the temperature difference between various regions of the ceramic. They are accentuated by the fact that the distribution of the power dissipation in the window is not necessarily completely radial. The stresses may generate cracks in the ceramic, or in the copper, or in the various brazed joints that provide a vacuum seal. The defects that may result from these thermal stresses are totally unacceptable with regard to the tube, once these defects result in a loss of vacuum sealing. This is why the level of power that can pass through the window during use of the tube has to be limited.
It is an object of the invention to produce a microwave window having higher power capabilities than in the prior art, while maintaining the advantages of the existing windows.